Accurate evaluation of visual function in young children in particular is challenging. There are few methods available and none are particularly suitable for use by health care providers who do not have extensive training in pediatric ophthalmology, optometry and/or visual electrophysiology.
Optokinetic nystagmus (OKN) is the phenomena of involuntary eye movement triggered by a continuously moving pattern or stimulus, and can be used to evaluate visual function. OKN is the repetition of an eye moving to smoothly pursue a target in a visual stimulus followed by a resetting event (saccade) where the eye fixes on a new target of the stimulus. FIG. 1(b) shows a graph of eye displacement over time showing a saw tooth OKN profile that is characteristic of OKN eye movement.
The presence or absence of OKN is an objective indication of visual performance and can also be useful for assessing neurological disorders. Early detection of a vision problem in a person's life is also known to significantly improve the outcome of proceeding treatment.
An OKN visual stimulus consists of an arrangement of lines or dots having modifiable properties including contrast, frequency or coherence. Manipulating these parameters allows measurement of the threshold at which OKN is no longer present. This threshold is a measure of visual performance.
One method of eliciting OKN is with use of a spinning hand-held drum that has a visual stimulus. A clinician watches eye movement to make a decision regarding the presence or absence of OKN.
Another method of detecting OKN is known as electro-oculography. Electrodes placed around the eye measure changes in electrical potential as the eye (a strong dipole) moves in response to a visual stimulus. The signals produced by the electrodes are analysed to determine the presence or absence of OKN.
Another method of detecting OKN is with the use of video-oculography techniques. The position of an eye is tracked by reviewing live or recorded video. Video-oculography is appealing because it is a non-invasive and can be implemented using low cost and basic video hardware. U.S. Pat. No. 7,380,938 describes a video-oculography technique where video footage of an eye is recorded, the vertical and horizontal movement of an eye isolated and displacement of the eye in the vertical and horizontal directions is measured. The displacement measurement can be analysed to determine the presence or absence of OKN patterns.
One disadvantage of video-oculography techniques is that they require a fixed positional relationship between the camera and eyes in order to eliminate performance errors introduced by head movement. To prevent head movement, the subject is often required to place their head in a fixed chin-rest or wear head mounted equipment such as cameras and lighting equipment to improve the video recording. Young children and infants do not tolerate head fixing or head-mounted gear which precludes them from an objective assessment of their visual function using video-oculography techniques.
Objects of the present invention relate to ways of assess optokinetic nystagmus which overcomes or at least ameliorates some of the abovementioned disadvantages or which at least provides the public with a useful choice.
In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.